Process of manufacturing a three-dimensional article

ABSTRACT

This invention involves a new and better solution to the problems associated with the premature softening of PLA filaments in the additive manufacturing of three dimensional articles. It is based upon the finding that poly(lactic acid) filaments with high crystallinity offer much better resistance to heat-induced softening. The crystalline poly(lactic acid) filament of this invention can accordingly be used in the additive manufacturing of three dimensional articles without encountering the problems associated with premature softening, such as poor quality and printer jamming. The crystalline poly(lactic acid) filaments of this invention can also be used in additive manufacturing of three dimensional articles without compromising the quality of the ultimate product, reducing printing speed, increasing cost, or leading to increased printer complexity. This invention more specifically discloses a filament for use in three-dimensional printing which is comprised of crystallized poly(lactic acid), wherein said filament has a diameter which is within the range of 1.65 mm to 1.85 mm.

This is a divisional of U.S. patent application Ser. No. 15/309,255,filed on Nov. 7, 2016, which is 371 of PCT/CN2015/078566, filed on May8, 2015. The teachings of U.S. patent application Ser. No. 15/309,255are incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

By definition “rapid prototyping” is a group of techniques that can beused to quickly fabricate a scale model of a physical part or assemblyusing 3-dimensional computer aided design (CAD) data. In rapidprototyping, construction of the part or assembly is usually done in anadditive, layer-by-layer fashion. Those techniques that involvefabricating parts or assemblies in an additive or layer-by-layer fashionare termed “additive manufacturing” (AM), as opposed to traditionalmanufacturing methods which are mostly reductive in nature. Additivemanufacturing is commonly referred to by the general public as “3Dprinting”.

According to ASTM Committee F42 on Additive Manufacturing Technologies,there are currently seven basic AM technologies: material extrusion,material jetting, binder jetting, vat photopolymerization, sheetlamination, powder bed fusion and directed energy deposition. The mostwidely used of these seven AM technologies is based on materialextrusion. While some variations exist, this technology generallyinvolves feeding a thermoplastic polymer in the form of a continuousfilament into a heated nozzle, where the thermoplastic filament becomesa viscous melt and can be therefore extruded. The 3-dimensional motionof the nozzle or the extruder assembly is precisely controlled by stepmotors and computer aided manufacturing (CAM) software. The first layerof the object is deposited on a build substrate, whereas additionallayers are sequentially deposited and fused (or partially fused) to theprevious layer by solidification due to a drop in temperature. Theprocess continues until a 3-dimensional part is fully constructed. Theprocess may also involve a temporary support material that providessupport to the part being built and is subsequently removed from thefinished part by mechanical means or dissolution in a suitable liquidmedium. This process is commonly referred to as fused depositionmodeling (FDM) or fused filament fabrication (FFF). This technology wasfirst described by the teachings of U.S. Pat. No. 5,121,329.

U.S. Pat. No. 5,121,329 more specifically discloses an apparatus formaking three-dimensional physical objects of a predetermined shape bysequentially depositing multiple layers of solidifying material on abase member in a desired pattern, comprising: a movable head havingflow-passage means therein connected to a dispensing outlet at one endthereof, said outlet comprising a tip with a discharge orifice ofpredetermined size therein; a supply of material which solidifies at apredetermined temperature, and means for introducing said material in afluid state into said flow-passage means; a base member disposed inclose, working proximity to said dispensing outlet of said dispensinghead; a mechanical means for moving said dispensing head and said basemember relative to each other in three dimensions along “X,” “Y,” and“Z” axes in a rectangular coordinate system in a predetermined sequenceand pattern and for displacing said dispensing head a predeterminedincremental distance relative to the base member and thence relative toeach successive layer deposited prior to the commencement of theformation of each successive layer to form multiple layers of saidmaterial of predetermined thickness which build up on each othersequentially as they solidify after discharge from said orifice; and ameans for metering the discharge of said material in a fluid stream fromsaid discharge orifice at a predetermined rate onto said base member toform a three-dimensional object as said dispensing head and base memberare moved relative to each other. In one embodiment of the inventiondescribed in this patent, the material is in the form of a continuousflexible strand.

Material extrusion based AM (FDM or FFF) has become quite popular overthe course of the past decade, largely due to the emergence of low-cost,desktop 3D printers. Such printers feature small sizes (similar todesktop inkjet printers) and are usually sold at a price of under $5,000(United States dollars) per unit. Examples of material extrusion baseddesktop 3D printers are Replicator® series 3D printers from MakerBotIndustries, H-series printers from Afinia, M-series printers fromMakerGear LLC, etc. Some of those 3D printers are based on open-sourcehardware and are available as DIY kits.

There are several thermoplastic polymers that are currently being usedin material extrusion based AM processes, such as FDM or FFF. Thosematerials include acrylonitril-butadiene-styrene (ABS), poly(lacticacid) (PLA), polycarbonate (PC), polystyrene (PS), high impactpolystyrene (HIPS), polycaprolactone (PCL), and polyamide as well assome other polymeric materials. However the most commonly used materialsare ABS and PLA.

ABS has the advantage of good overall mechanical properties; however itsuffers from relatively large volumetric shrinkage and the generation ofunpleasant odors. Furthermore, the generation of potentially toxicdegradation products during printing makes ABS a less suitable optionfor desktop 3D printers because such printers generally do not have aheated build envelope and an effective mechanism to eliminate the odorand toxic degradation products. PLA, on the other hand, has lessvolumetric shrinkage which allows it to be printed properly even withouta heated build envelope. It generates no unpleasant odor duringprinting, and the main degradation product is lactic acid which posesminimal health risk to 3D printer users. According to many surveys, PLAis increasingly becoming the most used material for desktop 3D printers.However, PLA still suffers from a number of drawbacks, including poorimpact strength and a low softening temperature. The low softeningtemperature leads to difficulties with extrusion and printing quality.Accordingly, some 3D printers employ PLA solutions onto a substrate toform a 3D object. Such PLA solutions harden due to the evaporation ofthe solvent, rather than because of a change in temperature. However, itis desirable to reduce the need for such solvents.

A schematic of a typical printer head or extruder used on a FDM/FFF 3Dprinter is illustrated in FIG. 1. During conventional use, a filament 1with an average diameter of d_(F) is moved by two counter-rotating feedrollers 2, subsequently into a filament barrel 3 with an inner diameterof d_(I) and a heater block 4. To function properly, the filament shouldremain solid in the filament barrel and only becomes a viscous melt inor close to the heater block section. The solid part of filament 1 inthe filament barrel 3 functions as a plunger that pushes the melt out ofthe nozzle 5. The nozzle orifice usually has a diameter in the range of0.2-0.5 mm, and more typically has an orifice diameter which is withinthe range of 0.3-0.4 mm.

During printing, heat can migrate from the heater block to the filamentbarrel. This can lead to premature softening of the filament in thefilament barrel, when the temperature of the filament barrel gets nearor higher than the softening temperature of the filament. This situationis schematically illustrated in the FIG. 2. The problem associated withpremature softening of the filament is commonly encountered inapplications where PLA filaments are utilized. In any case, when a PLAfilament softens in the barrel of a 3D printer it becomes highly viscousand “swollen” due to both polymer relaxation as well as the compressionby the portion of the filament that is still solid. This creates a largeresistance between the filament and the filament barrel, resulting ininconsistent feeding or even jamming of the printer/extruder. Accordingto many 3D printer users, printer/extruder jam is the most significantproblem associated with desktop FDM/FFF printers, and prematuresoftening is the most frequent cause of printer jams in machines thatuse PLA filaments. This is a particularly difficult problem in the caseof large parts that require long printing times (as temperature cangradually rise over time) and in dual-extrusion printing (as oneprinting head has to sit idle while the other printing head is working).

There are several known approaches to deal with the problems associatedwith the premature softening of PLA filaments. One such solution is toincrease the internal diameter of the filament barrel, d_(I), thereforeminimizing the heat transfer between the barrel wall and the filament.However, this approach leads to compromised printing quality because themore closely the filament diameter matches the internal diameter of thefilament barrel the higher the quality of the article. Another solutionis to add more cooling to the filament barrel by using an active coolingfan and/or a heat sink. However, this leads to more complexity and“bulkiness” of the printing head which in turn reduces printing speedand adds to cost. There is accordingly a need for an approach which doesnot compromise the quality of the ultimate product, reduce printingspeed, and increase cost, or lead to increased printer complexity.

SUMMARY OF THE INVENTION

This invention involves a new and better solution to the problemsassociated with the premature softening of PLA filaments in the additivemanufacturing of three dimensional articles. It is based upon thefinding that poly(lactic acid) filaments with high crystallinity offersmuch better resistance to heat-induced softening. This is becauseconventional filaments which are made with amorphous poly(lactic acid)begin to soften at temperatures which approach their relatively lowglass transition temperature (Tg) of 55° C. to 65° C. As theconventional filaments of amorphous poly(lactic acid) are heated thereis a gradual change in viscosity with increasing temperatures (i.e.viscosity decreases gradually with increasing temperatures). This is incontrast to a dramatic change in viscosity with increasing temperaturewhich can be realized by using the crystalline poly(lactic acid)filaments of this invention in additive manufacturing applications. Thecrystalline poly(lactic acid) filament of this invention can accordinglybe used in the additive manufacturing of three dimensional articleswithout encountering the problems associated with premature softening,such as poor quality and printer jamming. The crystalline poly(lacticacid) filaments of this invention can also be used in additivemanufacturing of three dimensional articles without compromising thequality of the ultimate product, reducing printing speed, increasingcost, or leading to increased printer complexity.

This invention more specifically discloses a filament for use inthree-dimensional printing, said filament being comprised ofcrystallized poly(lactic acid), wherein said filament has a diameterwhich is within the range of 1.65 mm to 1.85 mm. It also reveals afilament for use in three-dimensional printing, said filament beingcomprised of crystallized poly(lactic acid), wherein said filament has adiameter which is within the range of 2.75 mm to 3.15 mm. Thecrystallized poly(lactic acid) will typically have a degree ofcrystallinity which is within the range of 5 percent to 40 percent andwill commonly have a degree of crystallinity which is within the rangeof 10 percent to 30 percent. In any case, the crystallized poly(lacticacid) will typically exhibit virtually no heat of crystallization. Thecrystallized poly(lactic acid) will also typically have a melting pointwhich is within the range of about 145° C. to about 185° C. as shown bythe endothermic peak temperature as determined by differential scanningcalorimetry (DSC). In some cases the filament will have a diameter whichis within the range of 1.70 mm to 1.80 mm and in other cases thefilament will have a diameter which is within the range of 2.75 mm to3.15 mm and which is preferably within the range of 2.80 mm to 3.05 mm.

The subject invention also reveals a process for manufacturing athree-dimensional article by additive manufacturing which includesextruding a filament of poly(lactic acid) into a desired geometricshape, wherein the filament of poly(lactic acid) is crystallizedpoly(lactic acid).

The present invention further discloses a method for manufacturingcrystalline poly(lactic acid) filaments which are particularly useful inthe additive manufacturing of three dimensional articles, said methodincluding the steps of: (1) extruding molten poly(lactic acid) into theform of an amorphous filament, (2) collecting the amorphous filament ona spool to make a spool of amorphous poly(lactic acid) filament, and (3)heating the spool of amorphous poly(lactic acid) filament to atemperature of at least the glass transition temperature of thepoly(lactic acid) for a period of time which is sufficient tosubstantially crystallize the poly(lactic acid), and (4) allowing thespool of crystallized poly(lactic acid) filament to cool to ambienttemperature.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration showing a typical printer head or extruder asis used in additive manufacturing printers.

FIG. 2 illustrates the premature softening of a filament in the filamentbarrel of a printer used in additive manufacturing.

FIG. 3 provides the DSC of as-extruded and heat-treated filaments whichwere made in accordance with the techniques of this invention.

DETAILED DESCRIPTION OF THE INVENTION

Poly(lactic acid), which is sometimes abbreviated as “PLA”, is a highmolecular weight polyester which is synthesized by the polymerization oflactide monomer, which is a cyclic dimer of lactic acid, or2-hydroxypropionic acid. Lactic acid is a chiral molecule with twoenantiomeric forms, 1-lactic acid d-lactic acid. Typically 1-lactic acidand d-lactic acid are both present in PLA. The composition of 1- andd-lactic acid is critical in determining the crystallization behavior ofPLA, including the degree of crystallinity and crystallization kinetics.Most commercially available PLA has higher 1-lactic acid content. Whend-lactic acid content increases, the degree of crystallinity, meltingtemperature, crystallization rate all decrease. PLA will show verylittle tendency to crystallize when the content of d-lactic acid exceeds15%. The PLA for the current invention is preferred to have an 1-lacticacid content in the range of 85% to 100%. Examples of such PLA materialsare 2500HP, 4032D, 2003D, 4043D and 7001D from NatureWorks LLC.

Most PLA filaments used in FDM/FFF based 3D printing are produced bymelt extrusion. In the melt extrusion process, fully dried PLA pellets,along with other ingredients, are fed into a polymer extruder (eithersingle-screw or twin-screw) with a cylindrical die and continuouslyextruded. The extrudate is subsequently quenched/cooled and pulled by apuller to give the desired physical dimensions before being collected.The process can also include equipment such as melt or gear pumps (toensure a stable output), laser micrometers (on-line measurement of thephysical dimensions), etc.

Melt-extruded PLA often remains amorphous due to the slowcrystallization rate of PLA. Those PLA filaments currently available forFDM/FFF based 3D printing are amorphous or have negligiblecrystallinity, resulting in a low softening temperature in the range of55° C. to 65° C. (as dictated by the glass transition temperature). Oneway to induce crystallization in an extrusion process is to increase theorientation of the filament, which is generally done by using a largedraw ratio and/or a cold drawing step in the extrusion process. Howeverthis was found to be undesirable for the 3D printing application as highorientation leads to too much relaxation of the polymer that can alsocause the “swelling” issue as illustrated in FIG. 2.

This invention provides a novel method to manufacture PLA filaments withhigh degrees of crystallinity. The method of this invention involves thefollowing key steps: (1) manufacturing the PLA filament using themelt-extrusion process; (2) spooling the filament on a spool; (3)heat-treat the spooled PLA filament at a temperature which is within therange of the glass transition temperature (Tg) of the PLA to about 80°C. above the Tg of the PLA) for an extended period of time which issufficient to substantially crystallize the PLA in the filament.

As mentioned before, it is desired to have a relatively low draw ratioin the melt-extrusion process. Draw ratio, for simplicity, is defined asthe ratio of the diameter of the cylindrical die used on the extruder tothe final filament diameter. For example, if a 3 mm die is used tomanufacture a filament of 1.75 mm diameter, the draw ratio is3/1.75=1.714. Draw ratio affects both the heat-treat step (discussedlater) and the 3D printing process. It was found that the draw ratioshould be in the range of 1 to 3.5, and is preferred to be in the rangeof 1.1 to 1.75.

Before applying the heat-treat step, it is critical to have the filamentspooled. Spooling allows the filament to be under slight tension whichcan help maintain the correct physical dimensions and prevent too much“kinkiness” when heat is applied.

The heat-treat step is the step that imparts crystallinity to the PLAfilament. Annealing the filament at a temperature which is above the Tgof the poly(lactic acid) for an extended period of time allows the PLAto slowly crystallize. The temperature range suitable for this step isfrom the Tg of the PLA to 80° C. above the Tg of the PLA, and preferredto be in the range of 10° C. above the Tg of the PLA to 40° C. above theTg of the PLA. For instance if the PLA has a Tg of about 60° C., thepreferred heat-treat temperature will be in the range of about 70° C. to100° C. The choice of temperature should be high enough to allow enoughpolymer chain mobility for crystallization to occur, but not so high asto induce significant sticking or even melting of the filament. For PLAit was found that a temperature which is within the range of 70° C. to100° C. is a good general temperature range. The required time for theheat treatment depends on the temperature, and is recommended to be noless than 1 hour, and preferred to be 2 hours or more. For example, theoptimum heat treatment profile for a PLA filament produced using 4043Dfrom NatureWorks LLC was found to be 90° C. for 2 hours.

Nucleating agents may be used to expedite the heat-treat step, byincreasing the rate of nucleation for the crystallization process.Examples of nucleating agents are: talc, silica, graphite, clay,inorganic salts, organic metal salts, inorganic pigments (such astitanium dioxide or carbon black), metal oxides, amides, and esters.Such nucleating agents can accordingly be included in the PLA at a levelwhich is within the range of about 0.1 weight percent to about 2 weightpercent. In cases where nucleating agents are included they aretypically present at a level with is within the range of 0.5 weightpercent to 1 weight percent.

It is critical to maintain the physical dimensions unchanged or oflittle change before and after the heat-treat step. In addition to thetemperature, the draw ratio and spooling are both important. The drawratio should not be too large, as a large draw ratio was found to causetoo much kinkiness, change in diameter, and also sticking of thefilament. The draw ratio should be in the range of 1 to 3.5, and ispreferred to be in the range of 1.1 to 1.75. Spooling gives the filamentslight tension, without which the filament will become kinky and thedimensions can change significantly. Since most 3D printing filamentsare supplied in spools, it is also convenient to heat-treat the filamentin spooled form.

The PLA filament disclosed in this invention, unlike any other PLAfilaments made using conventional processes, exhibits high degrees ofcrystallinity. The degree of crystallinity can be characterized usingdifferential scanning calorimetry (DSC). In a typical DSC experiment, asmall (several mg) sample of the filament is heated at a constantheating rate, from ambient or sub-ambient temperature to a hightemperature that is higher than the Tm of the filament. The heat flowdata is collected and plotted against temperature. The degree ofcrystallinity can be calculated as:

${\chi\left( {100\%} \right)} = {\frac{{\Delta\; H_{m}} - {\Delta\; H_{c}}}{\Delta\; H_{f}} \times 100\%}$wherein ΔH_(m), ΔH_(c) and ΔH_(f) are the heat of melting, heat of coldcrystallization, and heat of fusion, respectively. ΔH_(m) and ΔH_(c) canbe determined by integrating the endothermic melting peak and theexothermic cold crystallization peak, respectively, on the DSC curve.ΔH_(f) is taken from literature as 146 kJ/mol (Polymer Data Handbook,Oxford University Press, Inc., 1999). The key features of the PLAfilament manufactured using the disclosed method are:

(1) The filament is fully crystallized and exhibit no or minimal coldcrystallization (ΔH_(c)˜0) (whereas conventional, amorphous PLA willexhibit cold crystallization);

(2) The filament exhibits a degree of crystallinity in the range of5-40%, more typically in the range of 10-30%.

The filament can be manufactured into almost any diameter. However themost commonly used diameters for 3D printing are about 1.75 mm and 3 mm.It is important for the diameter to have a small variation, as largevariations in diameter can lead to poor printing quality and feedingproblems. It is preferred for the filament to have a variation of lessthan ±0.1 mm.

The filament useful herein is a solid, crystallized PLA filament at roomtemperature and/or at the temperature it is loaded into the printerand/or the printing head. This contrasts with other printers whichemploy, for example, liquid printing solutions. Without intending to belimited by theory, it is believed that the solid PLA filaments hereinare able to be applied via, for example, direct mechanical pressure ofthe filament via the rollers 2 in FIG. 1 which then feed the filament tothe heater block 4. The PLA is then melted by the heater block 4 andextruded out of the nozzle 5 to form the printed object 6. The PLAextruded from the nozzle typically cools down immediately upon extrusionfrom the nozzle 5 so as to then solidify into the printed object 6formed from crystallized PLA. Accordingly, it is believed that due tothe high Tg and crystallinity of the present filaments, the printedobject 6 (formed from crystallized PLA) will more quickly reach thedesired hardness. Furthermore, it is believed that the use of such afilament will reduce premature softening of the filament in the barrelof the printer and swollen polymers in the barrel, so as to avoidincreased viscosity in the barrel. This in turn is believed to producemore consistent feeding of the PLA though the nozzle and tosignificantly reduce jamming of the printer/extruder. Without intendingto be limited by theory, it is also believed that the present inventionallows maintained and/or improved printing quality as the filamentdiameter may more closely match the internal diameter of the filamentbarrel. This also reduces the need for an active or passive coolingelement on the filament barrel, thereby reducing printer complexity.

The filament should be reasonably straight in order to feed properlyinto the printing head. As straightness or kinkiness is difficult todefine, here we use a practical testing method to verify thestraightness. The method involves passing the filament through a ringgauge with an internal diameter of d_(F)+0.15 mm (d_(F) being theaverage filament diameter) and a thickness of 8.5 mm at a speed of about50 meters/minute. If the filament has large kinks, it will not be ableto pass the ring gauge. These tests can be used as a quality assurancestep for the filament with it being important for the filament to becapable of passing through the ring gauge at a speed of 50 meters perminute without breaking.

In addition to PLA, the filament can contain other ingredients, such as,but not limited to: colorants, pigments, fillers, fibers, plasticizers,nucleating agents, heat/UV stabilizers, process aids, impact modifiers,and other additives.

This invention is illustrated by the following examples that are merelyfor the purpose of illustration and are not to be regarded as limitingthe scope of the invention or the manner in which it can be practiced.Unless specifically indicated otherwise, parts and percentages are givenby weight.

Example 1

PLA (grade 4043D from NatureWorks LLC) in the form of pellets wasextruded using a 30 mm single-screw extruder equipped with a gear pumpand a cylindrical die with a diameter of 2.25 mm to manufacture filamentwith a targeted diameter of 1.75 mm (draw ratio=1.286). The processingparameters are shown in Table 1. The extrudate was subsequentlywater-cooled, stretched by a puller to a final diameter of about 1.75 mm(continuously monitored by a dual-axis laser micrometer) and collectedas a continuous filament on a large spool. The collected filament,without any post-processing, is designated as the “as-extrudedfilament”.

TABLE 1 2 1 (com- 3 4 5 6 Gear (feed pression (metering (flange) (gear(gear 7 pump zone) zone) zone) entrance) pump) pump) (die) (rpm) 170° C.200 210 210 205 205 210 15

The filament on the large spool was then transferred to smaller spools.Each smaller spool contains about 750 grams of the as-extruded filament.The smaller spools loaded with as-extruded filaments were placed in aconvection oven at 90° C. for 4 hours and then cooled in air to roomtemperature. The filament was designated as “heat-treated filament”. Theas-extruded filament and heat-treated filament were passed through adual-axis laser micrometer to measure the diameter profile. The resultsare shown in Table 2.

TABLE 2 As extruded Heat-treated Average diameter (mm) 1.754 1.756Standard deviation (mm) 0.016 0.017 Maximum diameter (mm) 1.799 1.793Minimum diameter (mm) 1.729 1.730

As Table 2 suggests there was very little change in physical dimensionsbefore and after the heat-treat step. The heat-treated filament hasalmost identical appearance to the as-extruded filament, based on visualinspection. Both filaments can pass a ring gauge with an internaldiameter of 1.90 mm and a thickness of 8.5 mm at a speed of about 50m/min.

Example 2

The as-extruded and heat-treated filaments in Example 1 were tested on aDSC instrument (Q2000, TA Instruments). The samples were heated from 20°C. to 200° C. at a heating rate of 40° C./min. The results are shown inFIG. 3. The dashed and solid curves are from the as-extruded filamentand the heat-treated filament, respectively. Both samples show a glasstransition at about 65° C. The as-extruded filament displays a peak atTg due to physical aging. The main difference between the two filamentsis in the melting behavior. The as-extruded filament shows bothcold-crystallization and subsequent melting (see the inset in FIG. 3 fora magnified view of area in the dashed box). The degree of crystallinityis very low, i.e. <0.5%. Therefore the material remains almostcompletely amorphous. In contrast, the heat-treated filament shows nocold crystallization, indicating that the material had fullycrystallized. The degree of crystallinity of the heat-treated filamentis (calculated from the melting peak) about 15%.

Variations in the present invention are possible in light of thedescription of it provided herein. While certain representativeembodiments and details have been shown for the purpose of illustratingthe subject invention, it will be apparent to those skilled in this artthat various changes and modifications can be made therein withoutdeparting from the scope of the subject invention. It is, therefore, tobe understood that changes can be made in the particular embodimentsdescribed which will be within the full intended scope of the inventionas defined by the following appended claims.

What is claimed is:
 1. A method for manufacturing crystallinepoly(lactic acid) filaments which are particularly useful in theadditive manufacturing of three dimensional articles, said methodincluding the steps of: (1) extruding molten poly(lactic acid) into theform of an amorphous filament, (2) collecting the amorphous filament ona spool to make a spool of amorphous poly(lactic acid) filament, and (3)heating the spool of amorphous poly(lactic acid) filament to atemperature of at least the glass transition temperature of thepoly(lactic acid) for a period of time which is sufficient tosubstantially crystallize the poly(lactic acid), and (4) allowing thespool of crystallized poly(lactic acid) filament to cool to ambienttemperature.
 2. The method as specified in claim 1, wherein the spool ofamorphous poly(lactic acid) is heated to a temperature which is withinthe range of 55° C. to about 145° C. in step (3).
 3. The method asspecified in claim 1, wherein amorphous filament is drawn to a drawratio which is within the range of 1 to 3.5.
 4. The method as specifiedin claim 1, wherein amorphous filament is drawn to a draw ratio which iswithin the range of 1.1 to 1.75.
 5. The method as specified in claim 1,wherein the spool of amorphous poly(lactic acid) filament iscrystallized in step (3) to a degree of crystallinity which is withinthe range of 5 percent to 40 percent, and wherein said crystalizedpoly(lactic acid) has a melting point which is within the range of about145° C. to about 185° C.
 6. The method as specified in claim 1, whereinsaid crystalized poly(lactic acid) has a degree of crystallinity whichis within the range of 10 percent to 40 percent.
 7. The method asspecified in claim 1 wherein said crystallized poly(lactic acid)filament has a diameter which is within the range of 1.65 mm to 1.85 mm.8. The method as specified in claim 1, wherein said crystallizedpoly(lactic acid) filament has a diameter which is within the range of1.70 mm to 1.80 mm.
 9. The method as specified in claim 1 wherein saidcrystallized poly(lactic acid) filament has a diameter which is withinthe range of 2.75 mm to 3.15 mm.
 10. The method as specified in claim 1wherein said crystallized poly(lactic acid) filament has a diameterwhich is within the range of 2.80 mm to 3.05 mm.